Cardiac Tissue

At the cellular level, the main components of
the postnatal heart are cardiomyocytes, cardiac fibroblasts, endothelial cells
and vascular smooth muscle cells (Banerjee I, fusel JW, 2007). In a normal
adult heart, cardiomyocytes occupy most of the tissue volume, but they are only
30% of the total number of cells. The remaining 70% consists of
non-cardiomyocyte cell types, among which cardiac fibroblasts represent the
vast majority (Jugdutt, 2003).

The heart is organized into a complex array of
cellular and non-cellular components. Cardiomyocytes are diplayed in laminae
(AA Young, IJ Legrice, 1998), which in turn are organized in layers of
cardiomyocytes from two to five cells thick and surrounded by a network of
endomysial collagen. In the other hand, the fibroblasts form a network of
interconnected cells distributed among the endomysial collagen surrounding
groups of cardiomyocytes (Borg TK, Ranson WF, 1981 - Camelliti P, Borg TK, 2005
- Goldsmith EC, Hoffman A, 2004 - Young AA, Legrice IJ, 1998). The distribution
of endothelial cells and vascular smooth muscle is restricted to the
vasculature. Based on this organization, it has been estimated that all
cardiomyocytes would be connected directly to one to five cardiac fibroblasts
(Kohl P, Camelliti P, 2005). Direct intercellular contacts of fibroblasts seem
to be supported by at least two types of cell to cell connection molecules:
connexins and cadherins. In rats, cardiac fibroblasts are connected by both
cadherins and connexin-40, while connexin-45 serves as a connection between
fibroblasts and cardiomyocytes. However, the distribution of different
connexins may vary significantly between species (Banerjee I, K Yekkala, 2006 -
Camelliti P, Green CR, 2004). Additionally, integrins mediate the connection
between cardiac fibroblasts and the extracellular matrix.